Once you are interesting in how MIM parts are made in small size, sintered metal is the most essential area you should learn about. Every step in Metal Injection Molding process are traced back to the process of sintering. Sintering process is the most critical step in metal injection molding. It determines the properties and tolerance of final parts.
What is sintering
Sintering is the process of fusing fine particles together into solid mass by usage of combination of pressure and heat without melting materials. As in metal injection molding technology, our common particles are metal alloys including: stainless steel, low alloy steels, copper alloys, titanium alloys and special alloys. Sintering is applied to improve the strength and structural integrity of final MIM parts, it is the final step in MIM process.
Sintering process in Metal Injection Molding involve 4 steps:
Debinded parts are placed on ceramic trays to minimize movement in sintering, and then placed into high temperature furnace.
Brown parts are heated slowly in furnaces, and remaining binder is evaporated.
Heat parts in high temperature to fuse metal particles together as dense solid parts.
Cool the furnaces and remove sintered parts.
This sintering process will take anywhere form 15 to 20 hours. Any additional steps are post-operation process, like machining or surface coating.
Sintering furnace types
MIM sintering normally utilize two kinds of furnaces: continuous furnace and batch furnace
Continuous furnace: this furnace is able to debind and sinter in same step, its temperature can reach near to basic metal molten temperature. It is the best option for high volume MIM parts production.
Batch furnace: this furnace temperature also can reach to melting temperature of base metals, but its sintering process time is much less than continuous furnace.
All these furnaces operate under vacuum environment, during sintering process, flow gas is pumped into furnace to protect metal part from oxidation, de-carbonization or over-carbonization. These gas can be nitrogen, argon or hydrogen. There are also supporting equipment for furnaces, such as hydrogen generator, nitrogen generator, and emergency powder generator.
What is Sintered Metal Parts
Sintered metal are metal alloys powders throng sintering technology, as the details in micro aspect are quite complex, but the basic concept is easy to understand. As we familiar with sintering concept, it is easy to realize how metal injection molding can produce high strength parts with complex geometries, which are viable in various applications.
In order to create small size metal parts with complex geometries, you need to press them into required shape with other composition, then remove other binder materials and heat them at setting temperature. Metal alloys are stubborn, so sintering will improve final parts performance.
The sintering process in metal injection molding involves heating fine metal powder to fuse the particles, which give rise to final harder and stronger parts. Sintering is the final major step to produce MIM parts.
Preparation for MIM Sintering
In order to understand how sintering fits into metal injection molding, we need to start at the beginning of metal injection molding process:
1. Select perfect feedstock composition
In order to satisfy your MIM projects performance requirements, we need to provides the perfect feedstock with suitable metal alloys powder and binder materials. This will yield the right results of MIM parts physical and chemical properties.
The feedstock mix includes the basic fine metal powder you required, such as stainless steel, low alloy steel, nickel alloy, copper alloy, and titanium alloy, and special alloy (ASTM F15, ASTM F75, ASTM F1537). It also includes other substance as binder materials( polyethylene or polypropylene, wax and stearic acid), which normally be plastified and injection molded in high temperature.
2. Injection molding
Feedstock will be heated as melted toothpaste consistency in injection machines, and injected into molds through opening gates. After cooling process, we can get the green parts with corresponding three-dimensional structure. As green parts are consist of nearly 20% binder materials in volume, we cannot sinter them directly, additional process of debinding is critical before sintering.
Debinding utilizes various method as chemical or thermal ways to remove most of binder materials, in addition, left enough binder to hold parts size and geometry as backbone. We refer these parts as brown parts. As binder removal methods depend on binder materials composition, after debinding process, only little binder materials left to keep structure. These parts still need to finish by sintering.
Sintering is a process to achieve optimal strength and hardness for final MIM parts. In this process, brown parts are placed into furnaces with protective or vacuum atmosphere. Precise temperature profile is controlled and monitored to specific temperature, which is just below the melting point of primary metal. In order to remove remaining binder in early cycle and bond metal particles together.
Difference between Sintering and Melting
As people always confuse sintering and melting as similar, but there are significant differences. Melting actually heat the metal hot enough to change it from solid to liquid. While sintering is different because the heating process is controlled under metal molten temperature, and achieve enhanced properties as metal alloys. This encourage sintering to produce metal parts with high melting points easily.
Sintering process accomplish several stages with correct operation:
Eliminating remain binder materials in brown parts.
Reducing metal surface oxygen amount.
Developing sintered necks between metal particles, reducing surface pores and increase parts strength.
The last stage is key importance to sintering process, which determine the properties of final MIM parts.
Issue in Sintering
There are some issues during sintering process, so MIM design step is critical in our business plan. Our engineering team has ability of finding our all the potential defects or issues, in order to avoid problems in final parts. These issues include no consideration of gravity or friction affections, which will give rise to warping of final MIM parts. There are several options our engineers apply to minimize this issue, like adding support ribs, spacers or coining. Sagging is another issue in sintering, special setters can be used to support pieces area of most likely to sag upright, in order to solve this issue. In addition, we also can set a special ceramic tray with a runner to fix it.